High capacity fiber-optic transmission systems such as dense wavelength-division-multiplexed (WDM) and/or time-division multiplexed (TDM) systems with many closely spaced wavelength and/or time channels modulating at high data rate (>10 Gb/s) are required to meet the growing demand of Internet traffic bandwidth. For long distance fiber-optic links, accumulated optical noise from a chain of optical amplifiers and transmission impairments such as fiber nonlinearities and dispersion (chromatic and polarization mode) limits the practical transmission capacity-distance product. As the transmission distance improves with lumped and distributed optical amplifiers, accumulated nonlinear phase shift due to fiber Kerr nonlinearities becomes a primary concern for WDM/TDM channels with tight channel spacing. The detrimental nonlinearities include interaction of different channels through cross-phase modulation (XPM) and four-wave mixing (FWM). The maximum achievable transmission distance for a minimum bit-error-rate at the receiver typically balances the need to launch high optical power to reduce accumulated optical amplifier noise in order to achieve the required signal-to-noise ratio at the receiver with the need to constrain the optical power in order to mitigate fiber nonlinearities with a combination of dispersion and optical power management. Careful engineering design of the dispersion-managed transmission link with novel modulation techniques that resist fiber nonlinear impairments is essential.
Conventional modulation formats use amplitude shift keying or on-off keying (OOK) to encode binary information onto an optical beam. The result is an optical pulse pattern wherein the presence or absence of a pulse represents a logic ONE or logic ZERO. Currently, non-return-to-zero (NRZ) OOK is widely used in deployed commercial systems because it is relatively simple and low cost to implement and the technology is mature and compatible with industry standards. The width of an NRZ-OOK pulse is the same as the bit period. Another OOK data format is return-to-zero (RZ) that has a duty cycle less than 0.5 with a pulse width less than the bit period. The RZ data format is typically used in long- and ultralong-haul transmission systems, such as submarine systems.
Various forms of RZ-OOK format such as chirped RZ pulses with bit-synchronous sinusoidal phase modulation are described in U.S. Pat. Nos. 5,946,119 and 6,005,702. In U.S. Pat. No. 5,875,045, a technique is proposed to actively adjust the duty cycle of the RZ signal. U.S. Pat. No. 6,014,479 proposes to launch RZ-OOK pulses sequentially both in time and in wavelength.
The information capacity of DWDM systems can be enhanced by increasing the spectral efficiency. This can be achieved by reducing the channel spacing. This, however, increases the penalties for linear as well as nonlinear crosstalk between channels. The crosstalk can be mitigated using a spectrally efficient modulation format such as carrier-suppressed RZ-OOK format with reduced spectral width compared with conventional RZ-OOK format.
All of the above-mentioned RZ-OOK techniques, however, do not address effectively the critical problem of cross phase modulation impairment in fiber (intensity-dependent refractive index). As the FWM penalties are minimized by use of non-zero dispersion-shifted and standard single-mode fibers, the impact of XPM increases. The XPM effect coupled with chromatic dispersion of the fiber produces undesirable accumulated amplitude noise to the WDM/TDM channels from inter- and intra-channel interactions or crosstalk. One of the major noise components induced by XPM can be traced to the bit patterning effect of OOK format. Inherent to OOK format are missing pulses, which represent logic zeros, and due to the random nature of the data there are isolated pulse patterns and sequential pulse patterns. As different channels collide with each other, all channels may experience random pulse intensity pattern and therefore crosstalk noise induced by XPM.
The spectrum of a typical OOK optical signal contains a strong carrier component with weaker data sidebands. The strong carrier component provides very efficient FWM components with other channels. A carrier-suppressed signal has a reduced FWM effect. Further, the optical phase of the neighboring pulse of an OOK channel is typically the same (in-phase). As the neighboring pulses spread due to dispersion the trailing and leading edges of the neighboring pulses have different frequencies (colors). The neighboring pulse edges disperse and coincide in time and interact with each other, generating unwanted new frequencies through FWM. This intra-channel FWM effect tends to deplete and distort the pulses and generates unwanted noise-like pulses. The undesirable new frequencies contribute to amplitude noise of other channels.
It should be noted that the peak power of an OOK optical signal is equal to two times the average power of the signal divided by its duty cycle. The factor of two comes from the missing pulses of the OOK data sequence: there are on average 50% zeros. Since the nonlinear phase shift from the fiber Kerr nonlinearities increases with the peak power of the pulses the factor of two due to the missing pulses reduces the maximum launch power that could be used to reduce the accumulated optical amplifier noise. What is needed to mitigate the fiber impairments is therefore a method of transmission that is capable of suppressing the intra and inter-channel FWM, XPM, and bit patterning effects.
A phase shift key (PSK) optical signal is generated by sending a continuous-wave (CW) laser to a phase modulator driven by a source of electrical binary data. To demodulate and recover the PSK signal, the optical phase information is converted to amplitude modulation before detection by a photo-diode. For a PSK signal, homodyne or heterodyne detection is employed that requires a local laser oscillator at the receiver. The frequency and phase of the local oscillator needs to lock to the incoming PSK signal as well as matching the polarization states. A simplified approach is to use DPSK with a self-homodyne demodulator such as an apparatus described in U.S. Pat. No. 5,319,438. The self-homodyne detection does not require a local oscillator and therefore does not need for complex frequency, phase, and polarization tracking devices.
A major drawback of CW-modulated PSK and DPSK formats is the conversion of phase to amplitude modulation through fiber dispersion. Because of the continuous phase modulation of the CW optical beam by the electrical binary signal, fast transitions of the binary data produces undesirable phase modulation on the CW beam (frequency chirping) and therefore spectral spreading. As the phase modulated CW beam propagates through dispersive fiber, amplitude modulations at the transitions are generated. The amplitude modulation produces intra- and inter-channel crosstalk noise induced by intensity patterning effect as a result of XPM. Further, modulation instability of the CW-PSK signal in anomalous dispersive fiber can cause severe distortions of the PSK signal under high launching power. Transmission of PSK signal in low or zero dispersion fiber such as dispersion-shifted fiber reduces the phase-to-amplitude conversion impairment. This, however, limits the application of the CW-PSK signals since significant portions of deployed fibers are non-dispersion-shifted fibers. Further, CW-PSK signals can suffer large FWM penalties for DWDM transmission in low dispersion fiber. A recent experiment demonstrated OCDMA using quaternary PSK with RZ pulses. However, short optical pulses are required for coding. Because of the broad spectrum associated with short pulses the transmission distance is limited.